The invention relates generally to heating, ventilation, and air conditioning (HVAC) systems. More particularly, the invention relates to bypass ducts and associated bypass dampers that allow excess air in the HVAC system to recirculate. Still more particularly, the present invention relates to determining and using the actual flow rate of bypass air as a variable for use in controlling the bypass damper.
A conventional zoned central HVAC system includes an HVAC unit that conditions air (e.g., heats or cools the air, or otherwise improves comfort or health-related characteristics of the air such as by ventilation, filtration or humidity control), a supply duct that flows the conditioned air from the HVAC unit, and it may also include a return duct that provides air to the HVAC unit for conditioning. The supply and return ducts, if present, are split into two or more branches. Each branch delivers conditioned air to a zone (i.e., portion of the building) from the supply duct. If a return duct is present air is withdrawn from the zones and passes directly to the HVAC unit. Otherwise the air is returned to the HVAC unit by passing through the zones of the structure due to the location of the unit and its lower inlet pressure. Usually, each supply duct branch is fitted with one or more adjustable automatic dampers that independently control the flow rate (e.g., ft3/m, cubic feet per minute or CFM) of air flowing to its corresponding zone as directed by the HVAC controller based on the comfort needs of the occupants of each zone. For example, a damper may be adjusted between a fully open position, a partially open position, or closed position depending on the desired flow rate of conditioned air to be supplied to the corresponding zone.
Some conventional zoned central HVAC systems also include a bypass duct with a partially opened bypass damper that allows a portion of the total flow rate of conditioned air output by the HVAC unit, referred to as bypass air, to bypass all the building zones and recirculate back to the HVAC unit. The purpose of the bypass air is to provide a path for excess air in the system. Excess air typically occurs when the total flow rate of conditioned air generated by the HVAC unit is greater than the total flow rate of conditioned air needed by or allowed to flow to the zones. An excess air condition may occur because of overly restrictive supply ducts, return ducts, or branches thereof, or because of one or more zone dampers being partially or fully closed to reduce the flow of conditioned air into the respective zones. Undesirable effects of excess air include air noise, high pressure in the system, reduced total air flow or overly conditioned air (e.g., conditioned air that is heated to a high temperature or cooled to a lower temperature than during normal system operation).
The bypass duct and bypass damper provide a path for such excess air, which helps reduce and/or eliminate the aforementioned problems associated with excess air. However, too much bypass air (i.e., excessive recirculation) is also undesirable. For example, excess recirculation of cooled air could freeze coils in the HVAC unit, and excess recirculation of heated air could result in air temperatures that are sufficiently high to overheat the HVAC unit or trip protective controls and shut down the HVAC unit. Thus, the flow rate of bypass air must be limited. Typically, the flow rate of bypass air is limited by (1) a recommended maximum bypass air flow rate (cubic feet per minute or CFM) or (2) a recommended maximum recirculation percentage (i.e., maximum percentage of the nominal flow rate generated by the HVAC unit).
Unfortunately, the actual flow rate of bypass air in most conventional zoned HVAC systems is not known, and cannot be controlled because the bypass damper is not adjustable or it is adjusted solely based on pressure. Rather, most conventional bypass ducts and dampers are designed for a fixed maximum air flow rate at assumed, fixed conditions. For example, the bypass damper is selected to achieve recommended maximum bypass air flow rate or maximum recirculation percentage based on an assumed constant, fixed maximum air flow rate generated by the HVAC unit. However, in reality, the flow rate of bypass air on a given zoned HVAC system may vary greatly as the pressure differential between the supply duct and the return duct varies. The differences in air pressure in the supply duct and the air pressure in the return duct may vary greatly due to different modes of operation of the HVAC unit, varying duct restrictions (e.g., due to clogged filters, debris accumulation in the ducts, etc.), and adjustments in various zone dampers. Moreover, the air flow rate delivered by the HVAC unit may also vary for the same reasons. Due to differences between the actual flow rate of bypass air and the designed flow rate of bypass air based on assumed conditions, undesirable excess recirculation may occur.
Accordingly, there remains a need in the art for improved systems and methods for controlling the flow rate of bypass air in a zoned HVAC system. Such systems and methods would be particularly well-received if they allowed for adaptive control and adjustment of the flow rate of bypass air based on actual conditions in the HVAC system.